1. Technical Field
This invention relates generally to compounds having activity as PDE10 inhibitors, and to compositions containing the same, as well as to methods of treating various disorders by administration of such compounds to a warm-blooded animal in need thereof.
2. Description of the Related Art
Cyclic nucleotide phosphodiesterases (PDEs) are represented by a large superfamily of enzymes. PDEs are known to possess a modular architecture, with a conserved catalytic domain proximal to the carboxyl terminus, and regulatory domains or motifs often near the amino terminus. The PDE superfamily currently includes more than twenty different genes subgrouped into eleven PDE families (Lugnier, C., “Cyclic nucleotide phosphodiesterase (PDE) superfamily: a new target for the development of specific therapeutic agents.” Pharmacol Ther. 2006 March; 109(3):366-98).
A recently described PDE, PDE10, was reported simultaneously by three independent groups (Fujishige et al., “Cloning and characterization of a novel human phosphodiesterase that hydrolyzes both cAMP and cGMP (PDE10A),” J Biol Chem 1999, 274:18438-18445; Loughney et al., “Isolation and characterization of PDE10A, a novel human 3′,5′-cyclic nucleotide phosphodiesterase,” Gene 1999, 234:109-117; Soderling et al., “Isolation and characterization of a dual-substrate phosphodiesterase gene family: PDE10A,” Proc Natl Acad Sci USA 1999, 96:7071-7076). PDE10 has the capacity to hydrolyze both cAMP and cGMP; however, the Km for cAMP is approximately 0.05 μM, whereas the KM for cGMP is 3 μM. In addition, the Vmax for cAMP hydrolysis is fivefold lower than for cGMP. Because of these kinetics, cGMP hydrolysis by PDE10 is potently inhibited by cAMP in vitro, suggesting that PDE10 may function as a cAMP-inhibited cGMP phosphodiesterase in vivo. Unlike PDE8 or PDE9, PDE10 is inhibited by IBMX with an IC50 (50% inhibitory concentration) of 2.6 μM. (See Soderling and Beavo, “Regulation of cAMP and cGMP signaling: new phosphodiesterases and new functions,” Current Opinion in Cell Biology, 2000, 12:174-179.)
PDE10 contains two amino-terminal domains that are similar to the cGMP-binding domains of PDE2, PDE5 and PDE6, which are domains conserved across a wide variety of proteins. Because of the wide conservation of this domain, it is now referred to as the GAF domain (for the GAF proteins: cGMP binding phosphodiesterases; the cynobacterial Anabaena adenylyl cyclase; and the Escherichia coli transcriptional regulator fhlA). Although in PDE2, PDE5 and PDE6 the GAF domains bind cGMP, this is probably not the primary function of this domain in all cases (e.g., E. coli are not thought to synthesize cGMP). Interestingly, in vitro binding studies of PDE10 indicate the dissociation constant (Kd) for cGMP binding is well above 9 μM. As in vivo concentrations of cGMP are not thought to reach such high levels in most cells, it seems likely that either the affinity of PDE10 for cGMP is increased by regulation, or that the primary function of the GAF domain in PDE10 may be for something other than cGMP binding.
Inhibitors of the PDE family of enzymes have widely been sought for a broad indication of therapeutic uses. Reported therapeutic uses of PDE inhibitors include allergies, obtrusive lung disease, hypertension, renal carcinoma, angina, congestive heart failure, depression and erectile dysfunction (WO 01/41807 A2). Other inhibitors of PDE have been disclosed for treatment of ischemic heart conditions (U.S. Pat. No. 5,693,652). More specifically, inhibitors of PDE10 have been disclosed for treatment of certain neurological and psychiatric disorders including, Parkinson's disease, Huntington's disease, schizophrenia, delusional disorders, drug-induced psychosis and panic and obsessive-compulsive disorders (U.S. Patent Application No. 2003/0032579). PDE10 has been shown to be present at high levels in neurons in areas of the brain that are closely associated with many neurological and psychiatric disorders. By inhibiting PDE10 activity, levels of cAMP and cGMP are increased within neurons, and the ability of these neurons to function properly is thereby improved. Thus, inhibition of PDE10 is believed to be useful in the treatment of a wide variety of conditions or disorders that would benefit from increasing levels of cAMP and cGMP within neurons, including those neurological, psychotic, anxiety and/or movement disorders mentioned above.
While advances have been made with regard to inhibition of PDE10, there remains a need in the field for inhibitors of PDE10, as well as the need to treat various conditions and/or disorders that would benefit from the same.